FreeBSD's mkdir can't handle the sticky bit

According to FreeBSD's sticky(7):

  Neither open(2) nor mkdir(2) will create a file with the sticky bit set.

This issue should be handled by Go itself, but for the time being, I'd like
to keep the shim on Docker.

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
? :)